The invention concerns a machine for skiving or splitting nonmetallic planar workpieces, in particular plastic-coated paper-like workpieces, having a motor-driven moving knife, in particular a bell knife or hoop knife, whose cutting edge has a specific contour and moves continuously around a closed loop; and having a grinding device for grinding the knife.
A skiving machine of the aforesaid kind is known, for example, from DE 41 01 377.
Skiving machines are used, for example, in the shoe industry or pursemaking industry, to create a specific profile on the edges of cut sheets of leather, leather replacement materials, rubber, or plastic. A bell knife made of metallic material, which is cup-shaped and is equipped on its front circumferential edge with a cutting edge, is used for this purpose. The bell knife generally sits directly on the shaft of a high-speed, high-output electric motor, or on a spindle and is driven by a motor via a belt. Because the cutting edge of the bell knife made of metallic material is susceptible to wear, it is necessary to grind the cutting edge, from time to time or continuously, so as to sharpen it. Grinding devices that have corresponding deburring devices, as well as dressing devices for a grinding wheel of the grinding device, are known for this purpose.
A splitting machine is known, for example, from DE 38 15 130 A1.
This splitting machine is a hoop knife splitting machine that is equipped with a knife, in the form of an endless steel hoop, that runs over two belt pulleys, one of which is motor-driven. Since the circulating steel knife is subject to wear, a grinding device is provided with which the hoop knife is intermittently or continuously ground to sharpen it.
A feature common to both machines, i.e. skiving and splitting machines, is therefore the fact that they have a moving knife with a specific cutting edge contour that must be reground from time to time.
Since the contour of the cutting edge and the exact position of the cutting edge or cutting line is critical in terms of the skiving or splitting quality, the designs must be such that a flawless cutting edge contour is guaranteed despite the continuous or intermittent regrinding, and such that the position of the cutting edge or cutting line is exact and predetermined.
The field of application of such machines is gradually expanding, since skiving or splitting operations that earlier were performed principally on planar workpieces made of leather are now also being performed on other types of material.
Such workpieces include, for example, paper-like or board-like materials coated with plastic and/or aluminum foil.
Materials of this kind are used, for example, in the beverage industry. Beverage packages such as, for example, milk cartons or juice cartons are manufactured from plastic-coated board stocks of this kind.
Cut sheets of this kind can easily be coated on both sides with a plastic material that is approved for food contact, for example polyethylene. On the cut edges, however, the paper core material is exposed. In order to process these board materials into sealed beverage cartons, it has become known to equip the edges with, for example, a stepped cut by way of a skiving operation. This involves applying a corresponding stepped cut in the region of one edge of the cut sheet, on one side, using a bell knife of a skiving machine, so that a thinner portion of the board material remains on the rim which is then coated with plastic on only one side. This rim is then folded over itself, specifically over the uncoated side, thus resulting in an edge that is coated with plastic on its continuous peripheral edge as well. By heat-sealing several such folded-over edges to one another, it is then possible to construct a three-dimensional object, for example a parallelepiped or tetrahedron.
In processing such materials with a bell knife, it has been found that a deposit of contaminants, which is highly adherent and is made up predominantly of plastic coating material from the workpieces being processed, gradually accumulates in the region of the cutting edge. In the case of a skiving operation, for example, 560 meters per minute of edge material can be cut or skived.
Since the cutting edge contour of the knife is reground periodically or continuously, the grinding device encounters the adhering contaminants in the region of the contour and removes them as well.
Since these contaminants are made of sticky, viscous materials, they clog up the grinding wheel, so that it quickly becomes unusable. As a result, only relatively short service lives of one to two hours can be achieved, since the grinding device is completely clogged with the plastic materials after only a few grinding cycles.
Since the cutting edge of a bell knife is usually ground only on one side, only the contaminants adhering to that one side can be removed at all by the grinding wheel.
Similar impurity problems can also occur in splitting machines on hoop knives, since they have a similar cutting edge contour and similar problematic deposits can thus occur.
It is thus the object of the invention to provide a remedy here, and to improve a skiving or splitting machine of the kind cited initially in such a way that long service lives can be obtained for the knife and grinding device.
According to the present invention, the object is achieved in that a cleaning device which can be placed against the contour of the cutting edge, and with which the contaminants adhering to the cutting edge can be removed, is provided, the cleaning device being adaptable to the contour of the cutting edge.
The provision of a separate cleaning device has the considerable advantage that contaminants adhering to the cutting edge can be removed before the grinding device comes into engagement with the knife. This thus prevents the grinding device from quickly becoming clogged with the contaminants and requiring replacement.
The grinding device can then exclusively perform its grinding function, since it encounters a cleaned cutting edge; as a result, long service lives can be obtained for the grinding device, which then also results in long service lives for the knife.
Because the cleaning device is adaptable to the contour of the cutting edge, this contour is not adversely affected, i.e. its geometry is not changed and the cutting edge is cleaned exactly along its contour. This means that there are no remaining residues of contaminants that then are removed by the grinding device and would gradually clog it, which in turn would be detrimental to its service life.
Because the cleaning device is adaptable to the contour of the cutting edge, the grinding device is only just in engagement with the impurities on the cutting edge, and does not adversely affect the contour as such.
It has already been mentioned previously that the contour of the cutting edge is quite critical for the skiving or splitting quality. If residual contaminants were to remain behind, the grinding quality could be negatively influenced thereby, which is undesirable. In the case of thin knives, the geometry of the contour could be changed if the deposit were removed using large forces. The adaptability feature prevents this.
Very long service lives, and thus availabilities of up to 99.85%, can be achieved.
The object is thereby completely achieved.
In a further embodiment of the invention, the cleaning device is displaceable along the contour, transversely to the movement direction of the knife.
The advantage of this feature is that the cleaning device can be arranged and can work, in space-saving fashion, away from the actual working point of the knife on its edge and transversely to its movement direction.
In a further embodiment of the invention, the cleaning device cleans the cutting edge on both sides of its cutting portion.
The advantage of this feature is that the cutting edge is cleaned on both sides, thus continuously ensuring outstanding cutting quality.
If, for example, a knife is always reground only on one side, which is usual in the case of bell knives, the grinding device could in any case clean only one side, so that then when the aforementioned problematic materials are processed, the side opposite this reground side would gradually clog up with contaminants, which in turn would have a disadvantageous effect on grinding quality. During sharpening, a chip is detached from the material and is usually deflected and carried off via a chip deflector in the interior of the bell knife. If an increasingly thick deposit of contaminants then gradually builds up on the inner side of the bell knife, the chip can no longer be detached and carried off in precisely guided fashion.
In a further embodiment of the invention, the cleaning device has cleaning elements that can be guided back and forth along the contour transversely to the movement direction of the knife, and thereby detach contaminants adhering to the contour.
The advantage of this feature is that by way of the selection, number, and arrangement of the cleaning elements, flexible adaptation is possible to the local conditions of the skiving or splitting machine and to the type of contaminants, depending on which contaminant can best be removed by which cleaning element. The cleaning elements can then correspondingly be exchanged or replaced while the configuration of the cleaning device itself remains the same.
In a further embodiment of the invention, each cleaning element has an edge that peels off contaminants adhering to the contour.
This feature has the considerable advantage that the cleaning device operates as a peeling device, i.e. lifts off the contaminants, for example as chips, so that they are then carried off, without causing the cleaning elements of the cleaning device themselves once again to become gradually clogged or sticky.
The result can thus be, for example, that plastic material""s adhering firmly to a metal knife are peeled off and carried away as chips.
This not only increases the service life of the knife and the grinding device, but also yields a particularly considerable increase in the service life of the cleaning device, so that the overall result is a long service life for all three of these operating devices. The machine is almost continuously available for operation.
In a further embodiment of the invention, a cleaning element sits at one end of a lever that is pivotable against spring force and that can be pressed by the force of the spring against the contour of the cutting edge.
The advantage of this feature is to ensure, with mechanically simple and robust means, not only that sufficient contact pressure is present to clean the cutting edge, but also that, because of the pivotability of the lever, the cleaning element can be guided exactly and easily along the contour of the cutting edge.
In a further embodiment of the invention, two pivotable levers, which can be placed against opposite sides of the cutting edge, are provided.
The advantage of this feature is that each lever can be guided on its side exactly along the respective contour of the cutting edge on that side, i.e. even if different contours are present, for example in the case of knives that are asymmetrical or are ground on only one side.
In a further embodiment of the invention, a control system is provided which effects placement of the cleaning device against the knife only a certain short distance behind the cutting portion.
The considerable advantage of this feature is to ensure that the cleaning device does not come into contact with the highly sensitive outermost cutting edge which, as already mentioned several times, is indeed critical in terms of skiving or splitting quality. This prevents the cleaning device from bumping against the edge and thereby deforming it. In any case, relatively few contaminants are present directly at the tip, since the latter engages into the solid material without yet lifting a chip. It is in the regions behind the tip, i.e. in a region in which the chip to be detached is already lifting up slightly, that substantially greater volumes of contaminants become deposited over time. The tip is in any case removed in a subsequent grinding operation, and usually the opposite side is also processed in a subsequent deburring operation, so that no continuously occurring accumulation of contaminants is observed directly in the region of the tip.
In a further embodiment of the invention, two two-armed levers that are pivotable about a common lever axis are provided, and a spring, which presses one end of each lever respectively against one side of the knife edge, acts between the axes.
The advantage of this feature is that the two sides of the cutting edges can be cleaned, in a manner adaptable to the respective contours of the cutting edge, by way of physically simple and compact features that are nevertheless independent of one another.
In a further embodiment of the invention, the cleaning device has a linear drive by way of which the cleaning device can be displaced back and forth along the contour transversely to the movement direction of the knife.
The advantage of this feature is that the cleaning device can be moved selectably, i.e. periodically, cyclically, or continuously, back and forth along the contour, specifically transversely to the movement direction of the knife; in the case of skiving machines in particular, this can be embodied in very space-saving fashion.
In the case of splitting machines with long hoop knife travel lengths, it would also be possible to place the cleaning device against the edge in the movement direction, if allowed by the circumstances in terms of space.
It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention.